The invention pertains to a method for arc welding with a consumable electrode, wherein an alternating current is applied between the electrode and the work piece in order to generate the arc, wherein the alternating current has a current waveform that repeats itself after one period, and wherein the current waveform has at least one positive phase that is divided into at least one positive high-current phase and into at least one positive basic current phase and at least one negative phase within one period.
Different welding methods are employed in the arc welding under an inert gas atmosphere. In addition to methods with consumable electrodes which include the metal-active-gas welding and the metal-inert-gas welding methods, the tungsten-inert-gas welding method that is not carried out with a consumable electrode and the plasma welding method are also used. High-power welding methods have been increasingly utilized over the last few years in order to increase the productivity. High-power welding methods are usually carried out with consumable electrodes and are characterized by higher deposition rates than conventional metal-inert gas welding methods. The deposition rate is proportional to the wire diameter and the wire advance speed. The higher deposition rates can be converted into higher welding speeds and/or into higher welding seam volumes—in comparison with conventional welding methods. Fundamental principles of high-power metal-inert gas welding are described in greater detail in Merkblatt des deutschen Verbandes für Schweiβen und verwandte Verfahren e.V., DSV 0909-1 (September 2000) and DSV 0909-2 (June 2000).
During arc welding with a consumable electrode, an arc burns between the electrode being consumed and the work piece. An electric field is applied between the electrode and the work piece in order to generate the arc. The material transfer from the electrode being consumed to the work piece takes place by detaching drops from the electrode. If the electrode has a positive polarity, the detachment of the drops is effectively promoted by the pinch effect. The pinch effect causes the constriction of the drops being created at the end of the electrode such that the detachment of drops is simplified. This improves the process stability. If the electrode has a positive polarity, the direct current or the pulsed current technique is employed. If the electrode has a negative polarity, the arc climbs up on the electrode. This results in the energy transfer over a large surface that comprises the entire region at the termination of the electrode and therefore reduces the overheating at the electrode end. When using a negative polarity of the electrode, the detachment of the drops does not take place with the assistance of the pinch effect. The lacking constriction of the drops leads to process instabilities and to splattering during the material transfer such that welding with negative polarity is very rarely used for producing welded connections. In alternating current welding, a periodic reversal of the polarity of the electrode between a positive and negative polarity takes place.
Welding with an alternating current has been used for many years and this process forms the subject of various publications. For example, DE 4023155 describes a synchronization of the wire feed and the negative polarization of the alternating current. In DE 19906039, in contrast, a maximum current level for the positive phase is defined. EP 0890407 discloses a procedure for reducing the decay time during the drop of the positive welding current. EP 1491278 discloses the utilization of helium and of doped helium in the inert gas during alternating current welding. U.S. Pat. No. 6,376,802 describes a method that prevents an interruption of the arc. To this end, a first phase with positive polarity and a current that is sufficiently high for the detachment of the drops is followed by a second phase with negative polarity, wherein the current value of the second polarity no longer suffices for the detachment of the drops, and wherein the second phase is followed by a third phase with negative polarity and a current intensity, at which a detachment of drops does not take place.
However, significant splattering occurs at higher deposition rates in the welding with consumable electrodes and an alternating current. At high deposition rates, splattering is frequently so severe that welding is no longer possible because the material to be introduced into the connection or to be applied onto the work piece splatters into all directions and does not reach the processing point. However, this problem only occurs at high deposition rates because the splattering increases proportionally to the deposition rate. Consequently, the utilization of an alternating current welding method is either impossible or results in very inferior welding connections at high deposition rates.
High energy transfers are required in high-power welding in order to achieve high deposition rates. The energy associated with a high welding current leads to the softening and the detachment of the electrode material. However, at the end of the electrode that is in contact with the arc, this leads to overheating of the electrode material such that the energy supplied to the welding process cannot be utilized for the fusion process. In addition, process instabilities occur if the electrode becomes excessively hot.